Microwave ovens have been used to heat products for several decades. In their basic configuration, the microwave oven includes a magnetron which produces energy with a wavelength generally between 1 cm and 100 cm. In microwave ovens, the energy sent to the oven cavity preferably resonates in a plurality of modes that can be achieved by varying the dimensions of the oven cavity. These resonant modes cause the item to be heated by the rotation of the polar molecules (e.g. water) within the item. The absorption of the energy varies depending on the characteristics of the item as well as its size and shape. To ensure the uniformity of heating, it is desirable to vary the mode pattern with respect to the article by, for example, a mechanical mode stirrer which reflects the energy in different directions, by moving the item within the oven while the oven is operating, or by combinations of these two methods.
The internal cavity of the microwave oven is bounded by conductive side walls that confine the energy to the internal cavity. A door is included on one of the walls to provide access to the internal cavity of the microwave. Because of the door, several seams exist around the periphery of the door where the door meets the side wall. The plurality of electromagnetic modes within the oven can result in the propagation of energy having directional components along the seams which can produce undesirably large amounts of energy leakage through the seams. To alleviate this problem of leakage at the seams, seal devices have been developed to suppress the leakage of the electromagnetic energy. In fact, it would be difficult, if not impossible, for microwave ovens to obtain regulatory approval if an electromagnetic seal was not incorporated at these seams.
On basic household and commercial microwave ovens, the rectangular opening to the internal cavity is bounded by four flat walls which lie in the same plane as the opening. This can be visualized by having a rectangular side wall of the microwave oven with a smaller rectangular cut-out therein which provides an opening to the cavity. The portions of the side wall that surround the cut-out are these four flat side walls bounding the rectangular opening. The generally planar internal surface of the door is disposed in close proximity to those four walls when the door is in the closed position. The seams through which the electromagnetic energy can leak are defined by the internal surface of the door and the four flat walls against which the internal surface is positioned.
One basic way of suppressing the leakage of the electromagnetic energy uses an electromagnetic choke that is placed within the door. The choke includes a metallic panel that has a plurality of tabs or fingers which extend from the panel. The tabs are generally parallel to the seams defined by the flat walls of the oven and the door. The choke also includes a metallic structure, often referred to as a choke ring, positioned away from the tabs. The choke panel, choke ring, and the associated flat wall define a choke cavity which reflects energy propagating in a first direction back into the oven cavity and suppresses energy propagating in a second direction that is perpendicular to the first direction. Thus, the amount of electromagnetic energy that leaks from the oven is minimized. Such an electromagnetic choke is described in U.S. Pat. No. 3,767,884 to Osepchuk, assigned to the assignee of the present application, which is herein incorporated by reference in its entirety.
The frequency of energy that the choke cavity suppresses and reflects depends on the tab width, the tab spacing, the material of the tabs, and the overall dimension of the choke cavity. Thus, varying the physical characteristics of the choke panel and choke ring varies the frequency at which the choke will be effective. Consequently, the choke can be designed to act as an electromagnetic seal for the frequency at which the microwave oven is operating.
However, not all microwave ovens have seams that extend in the same plane as, or in a plane parallel to, the plane in which the opening to the oven cavity lies. A microwave oven may have a door which slides downwardly against a lower planar surface as opposed to a door that pivots around hinges. This may be the case if the microwave oven is automated and has a conveyer belt running therethrough for delivering the items that need to be heated to the internal cavity. When the seams around the door are not in the same plane, a standard planar choke panel cannot be used to suppress the leakage of the electromagnetic energy.
The effectiveness of a planar electromagnetic choke was believed to be reduced if its shape was deformed. One reason for this belief was that deformation of the planar electromagnetic choke creates additional reflective surfaces which would be transverse to the direction of propagation of energy that was to be suppressed. In other words, deforming the choke panel and the choke ring of a standard planar choke was believed to cause more resonation of the energy propagating in the direction that was supposed to be suppressed which leads to more leakage.
Accordingly, in many industrial microwave ovens where a conveyer carries the items into the oven cavity, the sealing of the oven cavity is provided by a suppression tunnel. The suppression tunnel is, in essence, a tunnel of conductive material that projects outwardly from the opening for a distance of several feet. Thus, one can look down along the length of the tunnel and see the oven cavity where the items are being heated; no structures inhibit such a viewing. The tunnel includes various structures (e.g. pins, corrugations, etc.) that attenuate the energy as it moves along the length of the tunnel. One problem with suppression tunnels is that they require much more space since the tunnels extend away from the microwave oven often for up to three or four feet. Another problem is that the opening to the oven cavity must be relatively small for the suppression cavity to be effective. Lastly, the suppression tunnel may be useful for industrial microwaves but cannot be used in commercial ovens since most regulatory agencies will not approve a commercial microwave oven that allows the operator to have a line of sight directly into the oven cavity.
Therefore, a need exists for an effective electromagnetic seal that allows for the sealing of a microwave oven which has seams that lie in different planes, as is the case for a microwave oven that operates on a conveyer system.